This invention relates to a total internal reflection (TIR) electro-optic modulator, and, more particularly, to a total internal reflection modulator for multiple axis and asymmetrical optical beam profile modulation.
Electro-optical devices have progressed in the art to provide a myriad of structures that operate on an electro-optic effect in a crystalline waveguide medium. Voltages are applied to an electrode array on the surface of the medium. These applied voltages cause the formation of a phase pattern in the medium which deflects and/or focuses the light wave propagating through the medium. This phenomenon occurs because an electric field, established in the medium by the voltages applied to the electrodes, varies the index of refraction of the medium sufficiently to change the relative phase of portions of the light wavefront as it propagates through the medium.
Total internal reflection electro-optic modulating devices have come into prominence and attention. In these devices, a set of interdigitated electrodes are employed on a major surface of the electro-optic medium to induce fringe electric fields into the electro-optic medium. Incident light is totally internally reflected from the major surface achieving interaction with fringe electric fields as the light passes near the surface of the material at close to grazing incidence.
More recently, it has been suggested to individually address alternate sets of electrodes in the total internal reflection electro-optic modulators. Voltages are applied to one set of interdigitated electrodes while the other set of interdigitated electrodes remain at a reference voltage level, such as ground, to produce a phase modulation of the light beam at the location of each addressed electrode. By applying a voltage difference to a pair of electrodes, the local magnitude of the electrical fringe field between them can be controlled and varied to permit deflection of the light to a predetermined point at an image plane.
Total internal reflection electro-optic modulators are used for wavelength separation of the incident light beam (U.S. Pat. No. 4,125,318), to form a diffraction pattern of a incoherent, unpolarized, nonconvergent, incident light beam (U.S. Pat. No. 3,958,862), as an electro-optic scanning device for deflection of a light beam across an image plane (U.S. Pat. No. 4,386,827, this patent being assigned to the assignee herein) or to convert an electronic signal pattern into a corresponding light intensity profile (U.S. Pat. No. 4,281,904).
It is known that changing an aperture size will change the profile of an incident light beam. Thus, tilting an aperture is a mechanical means of modulating a beam profile, and hence spot size of a light beam at an image plane. Typically, a sliding or rotating variable neutral density wedge is used to modulate the optical beam profile.
However, nonmechanical means, preferably electronic means, are needed to modulate an optical beam profile within an electronic optical device. The availability of electronic means to modify an optical beam will greatly increase the speed of response and thus the information bandwidth of the modulation channel. Electronic means of optical beam profile modulation will enable the potential of real time beam profile variation by computer control.
One recent total internal reflection modulator (U.S. Pat. No. 5,153,770) has the interdigitated electrodes of an electrode array extending inward from a rectangular-shaped outer electrode conducting block and outward from a diamond-shaped inner electrode conducting block. A diamond-shaped area with no electrodes is surrounded by the electrode array on the reflecting surface of the modulator. The uniform voltage difference between the electrodes and the varying lengths of the electrodes from the diamond within the rectangle pattern of the array will create a fringe electric field in the electro-optical material and an optical phase grating to diffract the incident light on the reflecting surface. The zero order nondiffracted light from the array and the reflected light from the non-electroded area becomes the modulated output beam. An alternate embodiment of that total internal reflection modulator has a diamond-shaped interdigitated electrode pattern within a rectangular shaped area with no electrodes. Using Schlieren optics, the non-zero order diffracted beam becomes the output beam with a modulated optical beam profile. However, these devices only modulate the optical beam profile along one axis and can only modulate the beam symmetrically.
It is an object of this invention therefore to provide a novel total internal reflection electro-optic modulator to modulate an optical beam profile along more than one axis.
It is another object of this invention to provide a total internal reflection electro-optic modulator to asymmetrically modulate an optical beam profile.
It is yet another object of this invention to provide a total internal reflection electro-optic modulator wherein the electrodes in a multiple electrode array are not individually addressable but the voltage for each set of electrodes is nonetheless varied.